change probe stats bitmaps to match %ju format

or should we use "%" PRIu64 in the format ?

reported by Travis via @fgsch

bin/varnishd/cache/cache_backend_probe.c

@@ -72,7 +72,7 @@ struct vbp_target {
 	unsigned			good;
 
 	/* Collected statistics */
-#define BITMAP(n, c, t, b)	uint64_t	n;
+#define BITMAP(n, c, t, b)	uintmax_t	n;
 #include "tbl/backend_poll.h"
 
 	double				last;